It is often necessary, or at least desirable, to concentrate a liquid mixture by removing a portion of the solvent, often water, from the liquid mixture. The resulting product, therefore, is in a more concentrated form.
It has been common to concentrate fruit and vegetable juices such as orange juice, grapefruit juice, grape juice, and tomato juice by evaporation to remove water. In addition, seawater and brackish water have been concentrated by evaporation, although the condensed vapor has been recovered as usable potable water rather than discarded as in concentrating fruit and vegetable juices. Nevertheless, each is a concentrating process. In the case of juice, the concentrate is the desirable product, whereas in obtaining potable water from seawater or brackish water the concentrate is discarded.
Evaporative concentration as described, as well as evaporation of chemical solutions or liquid dispersions, requires substantial energy since it relies on the latent heat of vaporization. Scaling of equipment and enhanced corrosion are often inherent at the temperatures involved in evaporative concentration. Loss of flavor and aroma also result during evaporative concentration of food products.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many products, particularly those having water as the liquid carrier. Generally, reduced energy is required since freeze concentrating relies on the heat of fusion instead of the heat of evaporation. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate in a concentrator or separator vessel. Next, the ice crystals are washed in a washer vessel to remove the concentrate remaining on them. The ice crystals can then be discarded or melted if potable water is desired.
Ogman U.S. Pat. No. 4,091,635 in part discloses freeze crystallizing in one vessel and then feeding an ice slurry to a wash column where the ice is separated and washed.
Nail U.S. Pat. No. 4,341,085 discloses freeze concentration apparatus which uses either a horizontal or vertical shell and tube freeze exchanger and a separate vessel for separating and washing the ice which is produced in the freeze exchanger. Engdahl U.S. Pat. No. 4,314,455 contains a similar disclosure.
In vertical shell and tube freeze exchangers, the top or upper ends of the spaced apart tubes are often made flush with the top surface of the upper tube sheet. Liquid fed to the top surface of the tube sheet often is not uniformly distributed to each tube mouth so that a constant falling film thickness and rate are not obtained. This adversely affects cooling efficiency. To improve liquid distribution to the tubes, the upper ends of the tubes are sometimes extended as, for example, about two to twenty-four inches above the tube sheet surface.
Another problem in operating a freeze exchanger of the vertical sheel and tube type is that a build-up of frozen solvent develops on the top end or edge of the tubes, and along the outer surface of the tubes when the tubes extend higher than the upper tube sheet. This build-up of frozen solvent, which is ice when water is the solvent or liquid carrier, can progress until liquid flow into the tubes is greatly retarded and even reduced to where nearly all flow is stopped. When this occurs the freeze exchanger must be taken out of operation and the frozen solvent melted to unplug the tubes. This represents a loss since the plant must be taken out of production for a substantial time.
The described build-up of solids is not limited to freeze exchangers but also can occur in heat exchangers operating at elevated temperatures and also in crystallizers in which a solute is solidified out of solution.
Clearly, a need exists for apparatus and methods which prevent or reduce build-up of frozen solvent, liquid carrier or a solute on the top ends of the tubes.